The present invention relates to a plasma display apparatus and a driving method thereof. More particularly, the present invention relates to an improved technique of a sub-field method for gradation display.
Plasma display panels (PDP) are known as image display apparatus to replace currently predominant cathode-ray tubes (CRTs). The plasma display apparatus has advantages of making it relatively easy to increase screen size and widen a viewing angle, having excellent resistance to environmental factors such as the temperature, magnetism, vibration and the like, having a long life, and the like. The plasma display apparatus is expected to be applied to wall-hung televisions for household use and to large information terminal apparatus for public use.
The plasma display apparatus applies a voltage to a discharge cell in which a discharge gas such as an inert gas is sealed in a discharge space, excites a phosphor layer within the discharge cell with vacuum ultraviolet rays generated from glow discharge in the discharge gas, and thereby obtains light emission. Thus, each individual discharge cell is driven on principles similar to those of a phosphor light. A large number of discharge cells are brought together to form pixels, whereby one display screen is formed. Each discharge cell is driven to be turned on or off, and thus produces two-gradation-step display in principle.
Plasma display apparatus are roughly classified into a direct-current driving type (DC type) and an alternating-current driving type (AC type) according to the method of applying a voltage to a discharge cell. The AC type plasma display apparatus is suitable for higher resolution because it suffices to form in a stripe manner barrier ribs serving to divide individual discharge cells within the display screen. In addition, since the surfaces of discharge electrodes are covered with a dielectric layer, the electrodes resist wear. The AC type plasma display apparatus therefore has an advantage of having a long life.
A sub-field method is known to realize multiple-gradation-step display on a plasma display apparatus that displays a screen by driving individual discharge cells on or off. In order to write and retain in a pixel multiple-gradation-step data formed by a plurality of weighted bits, the sub-field method divides one field into a plurality of sub-fields corresponding to the plurality of bits.
Each of the bits is written in the corresponding sub-field, and a driving signal corresponding to the weight of the bit is applied to the discharge cell to retain the bit. In other words, for a number of light emissions corresponding to the weight of each bit place of N-bit pixel data, the sub-field method divides the display period of one field into N sub-fields. The sub-field method thus produces display.
In a case of 8-bit pixel data, for example, the display period of one field is divided into eight sub-fields. In this case, the numbers of discharge light emissions of the sub-fields are set at for example 1, 2, 4, 8, 16, . . . , and 128, respectively, and 256-gradation-level display is produced by a combination of the eight sub-fields.
The number of discharge light emissions corresponds to the number of pulses included in the driving signal. Pulse frequency of the driving signal applied in each of the sub-fields is generally constant. A sub-field corresponding to a more significant bit has a large number of light emissions, and therefore has a long sub-field period.
On the other hand, a sub-field corresponding to a less significant bit has a small number of light emissions, and therefore has a small sub-field time width. In gradation display, in order to maintain brightness of the screen, all the sub-fields are set so as to be included in one field. With such setting, the less significant the bit, the smaller the time width of the sub-field.
In the case of a least significant bit, in particular, an effective time width included in the sub-field period is extremely short, and therefore it is difficult to produce a stable display.
There is a desire to increase the number of gradation steps for higher picture quality. When the number of gradation steps is thus increased, the number of sub-fields is also increased according to the number of gradation steps. Light emission sustaining periods of sub-fields corresponding to the less significant bit side are correspondingly shortened. There is also a desire to increase the number of scanning lines for higher picture quality.
When resolution is thus increased, the light emission sustaining periods on the less significant bit side are squeezed and shortened. In order to deal with such a problem, the pulse frequency of the driving signal tends to be raised so that sufficient brightness can be obtained even when picture quality is thus improved. However, simply raising the pulse frequency of the driving signal results in susceptibility to unstable operation, screen flicker, and inability to display correct gradation levels.